Many PNI researchers are now using natural killer cell cytotoxicity as an indicator of immune system functioning. According to Schulz et al.
), the natural killer cell assay is the immunoassay most likely to produce clear and replicable results in this type of research. Natural killer (NK) cells are lymphocytes which are classified as a non-B, non-T or null cells. NK cells are able to lyze tumor cell line in vitro
, while leaving normal cells intact (15
). This demonstrates the ability of NK cells to discriminate between normal and cancerous cells. Additional in vitro
data has demonstrated that NK cells are capable of killing a wide variety of cancerous cells including leukemia, carcinomas, sarcomas and melanomas (15
). Lower levels of NK cell activity have been correlated with the spread of cancer, demonstrated by increased tumor cell metastasis in both humans and animals (17
). In addition, the NK cell is a spontaneous killer, in that it does not have to be activated by tumor antigens, nor is it limited by major histocompatibility complex requirements (18
). There is also some evidence that natural killer cell activity is important in the prevention of some types of viral illnesses, most notably those induced by herpes simplex type 1, Epstein Barr and influenza viruses (19
Cardiovascular patients in Japan demonstrated a relationship between various psychological measures and immune function. A positive correlation was noted between NK cell activity and scores on both the extraversion scale and a sense of humor scale. They also reported a negative correlation between NK activity and scores on the neuroticism scale. The authors stated that the results indicate an association between general positive feelings and higher levels of NK cell activity. However, it should be noted that this work was correlational and did not examine cause and effect (21
A few researchers have attempted to document the influence of humor and/or humor response on NK cytotoxicity. With 22 breast cancer patients as the sample in a randomized crossover design (22
), all subjects viewed both a humorous video and a distressing video. NK cell cytotoxicity and numbers of NK cells were used as outcome measures. Half of the subjects viewed the humorous video first, while the other half viewed the distressing video first. Blood samples were taken from an indwelling catheter before and after each video, at 30 min following the video, and 23 h following the first video (just prior to viewing the second video). Sense of humor was also included and measured using the Coping Humor Scale (CHS). Scores on the CHS were positively correlated with numbers of NK cells, which would indicate that subjects with higher coping humor scores also had more NK cells. However, contrary to what was expected, no significant changes were noted in NK numbers or cytotoxicity at the end of either video or during recovery.
This unpublished dissertation also included an unexpected finding that NK cytotoxicity was significantly decreased 23 h after viewing the humorous video and was significantly increased 23 h after viewing the distressing film. Wise suggested that this could be a rebound effect, from a change in NK cytotoxicity (in the expected direction) sometime in the preceding 22 h. However, it should be noted that the 23 h post-stimulus test point was immediately prior to viewing the second video. Perhaps this result was related to anticipation of the upcoming video. This is a confounding factor that needs to be avoided in future humor research. In addition, as NK level is known to be reduced in patients with cancer, using cancer patients as the subjects in this investigational work may have lead to an additional confounding factor.
Another investigation concerning the effect of humor on NK cell activity used 10 healthy male subjects (5
). The experimental group viewed a 60 min humorous video, while the control group watched a non-humorous video. Blood samples were drawn from an indwelling catheter at baseline, during the movie, and 30 min after the movie (recovery). The experimental group displayed significantly increased NK cell activity during and following the humor video, from baseline to recovery (P
< 0.008), while the control group did not. However, this study involved a very small sample and was published in abstract form only.
In what is probably the most comprehensive report thus far, Berk (23
) reports the results of a series of five separate studies. The results are from a total of 52 healthy males, who viewed a humor video for 1 h. Blood samples were taken 10 min before, 30 min into, 30 min after and 12 h after the intervention. Increases were found in NK cell activity (P
< 0.01) and immunoglobulins G (P
< 0.02), A (P
< 0.01) and M (P
< 0.09), with several immunoglobulin effects lasting at least 12 h. The authors conclude that modulation of neuroimmune parameters by laughter may have indications for use in health and wellness intervention programs as adjunct to other integrative medicine therapies (23
Most studies that document a positive effect of laughter on NK activity have used all male samples. The use of all male subjects in studies using immune or endocrine-based outcomes simplifies the methodology, as it negates any possible effects that may occur due to female hormonal changes. However, it then becomes difficult to determine how a particular intervention may work in women. Because of this, a later study by our group used a sample of 33 healthy adult women to examine the effect of laughter on stress and NK cell activity (24
). This controlled study was designed to test the effect of humor on NK function, and to determine if sense of humor (measured by two different instruments) or laughter (measured by the Humor Response Scale—HRS) is related to change in stress NK activity when subjects are exposed to humor. Subjects were randomly assigned to view either a humor video or a neutral control video, with pre and post measures of stress and natural killer cell activity measured in all subjects.
Watching a humor video decreased stress for subjects in the humor group, as compared to those in the distraction group (P < 0.01). The effect of humor was apparently mediated by humor response, as amount of mirthful laughter correlated with the decreased stress seen in subjects who viewed the humor video (P < 0.01). Humor response also played a role in change in NK cell activity following the video. Subjects who scored less than 25 on the Humor Response Scale (indicating laughter) had increased immune function compared with their own pre-intervention immune function (P < 0.05), and compared to those participants who viewed the humor video but scored below 25 on the Humor Response Scale (P < 0.05). This finding indicated that only the subjects who laughed out loud during the humor video had significantly increased immune function following the intervention. Persons who just smiled or did not have observable responses to the humor video did not have positive changes in immune function. This finding is demonstrated in , indicating a significant relationship between the amount of laughter and change in immune function for persons in the humor group.
Figure 1. Plot represents the relationship between change in NK cell functioning and subject humor response (24).
Similar to findings from other studies, there was no connection between sense of humor (as measured Situational Humor Response Scale and the Multidimensional Humor Scale) and change in any of the study outcomes. Sense of humor was not related to change in stress, change in NK activity or even observed laughter in response to the humor stimulus. This finding supports earlier work (8–10
) that suggest that sense of humor (as measured by currently available instruments) does not appear to play a significant role in the physiological response to a humor stimulus. Findings from this study support that the act of laughing is correlated to changes in stress and immune function following exposure to a humor stimulus. Laughing can apparently reduce stress and improve NK cell activity, at least temporarily. As low NK cell activity is linked to decreased disease resistance and increased morbidity in persons with cancer and HIV disease, the authors conclude that laughter may be a useful cognitive-behavioral intervention for use in these patients. As an interesting side note, the differentiation in the effects of smiling versus laughing out loud was noted in another study of laughter, which examined the effect of smiling, laughing and howling on mood changes. It was found that howling did not substantially improve mood but both smiling and laughing did. Moreover, laughter seemed to boost positive affect more than just smiling (25
). Additional support for the effect of laughter on various physiological outcomes is briefly summarized in .
Brief overview of outcomes using a humorous stimulus